1. Field of the Invention
The present invention relates to a luminescent display device for giving a luminescent analog display of information inputted.
2. Description of the Prior Art
As is well known, the methods of displaying the results of various measurements, data processing and the like are usually divided into two types, namely digital and analog. When it is desired, for instance, to make a comparison of various quantities displayed or to obtain the ratios or changes of these quantities with respect to the whole, it is more advantageous to use the analog method than to use the digital one.
Meanwhile, with the recent startling progress of electronics, electronic or electro-optical display devices using light-emitting diodes, liquid crystals and the like have been put into practical use, being used as analog display devices in replacement of those of the conventional mechanical type.
Luminescent display devices, which are included among these electronic or electro-optical display devices, have advantages in that they are excellent in luminous color, capable of being driven at low voltage, low in power consumption, low in production cost, and so on. Therefore, it is expected that these luminescent display devices will find use as the above-mentioned analog display devices.
Luminescent display devices for analog display are usually driven either by the dynamic method or by the static method. The construction of the luminescent display device adapted to be driven by the static method is shown in FIG. 1 by way of example. The features, operation, advantages and disadvantages of the prior art will be hereinafter described with reference to FIG. 1 together with FIG. 2 which shows the behavior of the display given by the conventional luminescent display device.
In FIG. 1, reference numeral 1 designates a substrate on which a plurality of segment anodes 2(2.sub.1, 2.sub.2, . . . , 2.sub.n) each coated with a luminescent material layer are arranged either rectilinearly or curvilinearly. The segment anodes 2 are coupled to connecting terminals 3(3.sub.1, 3.sub.2, . . . , 3.sub.n), respectively. A filament-shaped cathode 4 (hereinafter referred to as filament) is provided above the segment anodes 2, emitting thermions when heated. A front bulb (not shown) is bonded to the substrate 1 along the peripheries thereof to form a casing so that the inside of the casing may be kept at high vacuum and the connecting terminals 3 of the segment anodes 3 and those (not shown) of the filament 4 may be airtightly extended through the front bulb.
With the above-mentioned construction, the conventional luminescent display device is operated as follows:
First, the filament 4 is energized while the segment anodes 2 are selectively given an anode voltage through the connecting terminals 3 according to the input information to be displayed. Thus, thermions emitted from the filament 4 are made to impinge on the segment anodes 2 to which the anode voltage has been given, and thereby a bar-shaped luminescent display is obtained the length of which corresponds to the above-mentioned input information.
In the foregoing display device shown in FIG. 1, the segment anodes 2 to which the anode voltage is not given are adapted to be given a potential equal to or lower than that of the filament 4 so as to prevent leakage emission.
Therefore, if the segment anodes 2.sub.1, 2.sub.2 and 2.sub.3 are given an anode voltage while those 2.sub.4 to 2.sub.n are kept negative with respect to the filament 4 as shown in FIG. 2 for instance, the path of the thermions impinging on the segment anode 2.sub.3 positioned at the boundary of the light-emitting region tend to bend by the action of a negative field formed by the segment anode 2.sub.4 with no light emission, and as a result an area S on which thermions do not impinge is created on the periphery of the segment anode 2.sub.3.
In other words, the electric field is disturbed in the vicinity of the above-mentioned boundary of the light-emitting region and as a result an area S where light is not emitted is formed on the segment anode 2 which is expected to emit light. Thus, the area S becomes a defect of display, or a so-called "shade" which makes the luminescent display unclear. Moreover, if the segment anodes 2 are strip-shaped and arranged at right angles to the filament 4, the segment anode positioned at the boundary of the light-emitting region does not emit light thereby causing a read error.
For the purpose of preventing the above-mentioned "shade", there has been proposed a display device in which a control grid in the form of, for instance, a net is provided between the filament 4 and the segment anode 2, being made positive with respect to the control grid to correct the disturbance of the electric field caused by the segment anode 2 which does not emit light so that thermions emitted from the filament 4 may impinge uniformly on the segment anodes 2 to which the anode voltage is applied.
In such a display device, however, the control grid provided between the filament 4 and the segment anode 2 prevents clear observation of the segment anode 2 which is emitting light, being expensive and complicated in construction. Therefore, such a conventional display device has problems in terms of performance, price and production.